1. Field of the Invention
This invention relates to a phosphate-free machine dishwashing composition. More particularly, this invention relates to a machine dishwashing composition which is free from phosphorus and which more efficiently removes food soils from cooking and eating utensils with less spotting and at least equivalent clarity to glassware and dishes when compared to conventional phosphate-built dishwashing compositions.
Mounting legislation outlawing the use of phosphorus in all household detergents has created considerable demand for phosphorus-free machine dishwashing compositions. However, despite significant efforts, no fully satisfactory phosphate-free machine dishwashing composition has, as yet, been obtained that is now commercially available.
In the detergent industry, distinctions are drawn between cleaning compositions on the basis of their functional utility; for example, those in the art are well aware that there are considerable art-recognized differences between cleaning compositions that are used for laundering purposes; cleaning compositions that are used for machine dishwashing purposes; and cleaning compositions that are used for hand dishwashing purposes. Generally, cleaning compositions for laundering purposes employ high foaming organic surfactants as the main cleansing agents. Foaming, unless it is excessive to the extent that it causes overflow from the washing machines, is generally considered beneficial in laundering compositions since it helps to loosen the soil from the fabric substrates by its local mechanical action. This local action is desirable since the substrate in the laundry machine receives comparatively poor mechanical agitation. By way of contrast, machine dishwashing methods that are currently used to wash china, glass, porcelain, ceramics, metal, and hard synthetic articles result in there being a high mechanical impact of the wash liquid that is sprayed onto the articles to be cleaned. Recently, developments in dishwashing apparatus have been directed toward further increasing the intensity of liquid motion as well as the water volume cycled per minute, and in this way to further improve the mechanical cleansing effect of the cleansing solution. Compared to laundering compositions, however, machine dishwashing compositions are very low-foaming compositions and preferably so, since foam formation reduces the mechanical impact of the liquid sprayed onto the articles to be cleaned. The surface active agents useful for machine dishwashing compositions should not only be low foaming materials in and of themselves, but they should also preferably be foam depressants, so that the foaming caused by protein and food residues in combination with alkaline cleansing solutions is kept to a minimum. Accordingly, the surfactant content of machine dishwashing compositions is very low. This situation, however, is quite different from hand dishwashing compositions, which, preferably, are high foaming and have more the attributes of laundering compositions.
2. Description of the Prior Art
Heretofore, the machine dishwashing detergent compositions that have been commercially employed in the art have been based on the use of phosphorus compounds that are now strenuously objected to on ecological grounds. For example, U.S. Pat. Nos. 3,579,455 and 3,627,686 require, respectively, the presence of tetra(alkali metal) pyrophosphate and an alkali metal hexametaphosphate, or the presence of tetra(alkali metal) pyrophosphate and alkali metal nitrilotriacetate.
In order to circumvent the phosphate requirements of the foregoing patents, later patents such as those of U.S. Pat. Nos. 3,700,599 and of 3,706,672 have found need to resort to the use of polymeric chelating agents per se (U.S. Pat. No. 3,700,599), or of such chelating agents in combination with an alkaline detergent salt or salts (U.S. Pat. No. 3,706,672). However, the amount of the polymer that is used in these patents is directly controlled by the degree of hardness of the water in which the dishwashing composition is to be utilized; for such amount has to be sufficient for purposes of chelating both the calcium and magnesium ions that are present. Thus, the primary function of the maleic anhydride/vinyl acetate copolymer of these latter two patents is to soften the water in which the dishes, glassware, etc. are to be washed by sequestering those metal cations which cause the hardness of such water. But this requires, for relatively hard water having 300 ppm or higher of those cations causing such water to be hard, a high polymeric or polyelectrolytic concentration to be present in the composition of the ultimate dishwashing product that is employed.
Use of strong chelating agents, however, such as the hardness sequestering polyelectrolytes discussed above as well as those now known and available to the art, pose, from an ecological standpoint, long range and unknown potential toxicological sources of danger. For example, strong sequestrants such as nitrilotriacetate, sodium oxydiacetate, etc., have been shown to be ecologically unacceptable owing to their potential toxicity; for it is said that these sequestrants can complex with heavy metals such as Hg.sup.++ and other trace heavy metals and offset the biological transport mechanism of these trace metals in living tissues. Thus, nitrilotriacetate has been blamed for causing fetus abnormality by a similar mechanism. By way of contrast, however, inefficient metal complexing agents are free from this drawback and this is believed to be the case with the polyelectrolytes contained in the present machine dishwashing compositions.
Contemporary ecological considerations, moreover, in another germane aspect, require that such formulated products as machine dishwashing compositions be at least 90% biodegradable. However, past and present machine dishwashing compositions now available to, and known in and by, the art, in order to be effective, must contain more than 10% polyelectrolytes and as such would not meet the stringent ecological requirements. Furthermore, high molecular weight polyelectrolytes such as those now used in the art are known to be biorefractory.
By way of contrast, and in resolution of the foregoing art-recognized deficiencies and needs, the present invention now provides a machine dishwashing composition that is able to function satisfactorily with low polyelectrolyte concentrations, i.e., polyelectrolyte concentrations so low as to fall far short of the amounts required to sequester the metal cations that cause the hardness of the water present in the wash solution. In fact, the most effective polyelectrolytes of the present invention are characterized by such low molecular weights and such a low degree of polymerization that they would be regarded as poor prospective candidates as sequestering agents consistent with the needs and requirements of U.S. Pat. Nos. 3,700,599 and 3,706,672. It has been found, by the present invention, that the select use of certain polyelectrolytes of a specific character and description in combination with an alkali metal, or ammonium, carbonate, whereby the weight ratio of polyelectrolyte to carbonate is kept within controlled limits, enables the resultant dishwashing composition, which comprises those two materials as essential ingredients, not only to function at least as efficiently and effectively as previous dishwashing compositions (such as those discussed above), but also to be operable (with less cost of manufacture) with polyelectrolyte levels previously found to be inoperable by the prior art.
In conclusion, it can be stated that there are two major differences between the present non-phosphate machine dishwashing compositions and those low- or non-phosphate polyelectrolyte-built machine dishwashing compositions that have been heretofore known or used in the machine dishwashing art, and these differences can be briefly summarized as follows:
(1) the present machine dishwashing compositions employ low concentrations of polyelectrolytes, whereas those of the prior art utilize high concentrations of polyelectrolytes.
(2) the present machine dishwashing compositions employ polyelectrolytes of low molecular weight which have very poor metal ion sequestering capacity, whereas those of the prior art utilize polyelectrolytes that are strong sequestrants for metal ions.
As has been seen, these two major differences to at least a substantial extent are believed tied to the surprising ecological advantages that accrue to the present machine dishwashing compositions vis-a-vis those of the prior art. As but a simple illustration of this, it can be noted that the present compositions comprise mostly inorganic salts such as an alkali metal, or ammonium, carbonate and contain less than 10% polyelectrolytes of low molecular weight, and this enables the resultant or ultimate machine dishwashing composition or product to be at least 90% biodegradable, even in the event the polyelectrolytes thereof may be biorefractory.